Spray deposition method for inorganic nanocrystal solar cells

ABSTRACT

A method of spray deposition for inorganic nanocrystal solar cells comprising subjecting a first solution of CdTe or CdSe nanocrystals to ligand exchange with a small coordinating molecule, diluting the first solution in solvent to form a second solution, applying the second solution to a substrate, drying the substrate, dipping the substrate in a solution in MeOH of a compound that promotes sintering, washing the substrate with iPrOH, drying the substrate with N 2 , and heating and forming a film on the substrate. An inorganic nanocrystal solar cell comprising a substrate, a layer of metal, a layer of CdTe, a layer of CdSe, and a layer of transparent conductor. An inorganic nanocrystal solar cell comprising a transparent conductive substrate, a layer of CdSe, a layer of CdTe, and a Au contact.

REFERENCE TO RELATED APPLICATION

This application is a non-provisional of, and claims priority to and thebenefits of, U.S. Provisional Patent Application 61/726,646 filed onNov. 15, 2012, the entirety of which is hereby incorporated byreference.

BACKGROUND

This disclosure concerns a method for the fabrication of a solar cell onan opaque, non-conductive solid substrate, where all of the componentsof the device can be deposited using a spray-based solution process.Critical elements are found in both the method of deposition as well asin the unique architecture of the cell.

Solution synthesized inorganic nanocrystals are generally composed of aninorganic core surrounded by an organic ligand shell, and each of thesecomponents performs a distinct role in device fabrication.

The inorganic core provides electronic function and an opportunity toexploit quantum confinement effects not seen in bulk inorganicmaterials. For semiconductors, this occurs when the nanocrystal diameterfalls below the Bohr exciton radius of the material.

The organic ligand shell stabilizes the core and enables thenanoclusters to dissolve in organic solvents, providing a practicalmeans for the solution processing of inorganic devices.

One area where these materials are currently under intense examinationis in the field of photovoltaics (PV), where this combination ofelectronic tuning via quantum confinement and solution processabilityhold promise for the fabrication of large area, flexible, and low-costdevices.

Initial approaches to the incorporation of nanocrystals intophotovoltaics involved dispersing the material into a conductive polymermatrix. In this configuration, the nanocrystals absorb visiblewavelength photons entering the active layer and consequently generatean exciton. Separation of this exciton into an electron and hole isaided by the polymer, serving as an electron transporting layer.

These early designs were plagued by low efficiencies and airsensitivity. In 2005, an all-inorganic design was first reported basedon the heterojunction formed between layers of CdTe and CdSe nanorodsdeposited through a spin coating process and illustrated in FIG. 1. Notethat this device contains no polymer in the region between nanorods,relying instead on the nanomaterial itself to generate and transportcarriers. In addition to its improved power conversion efficiency of2-3% under air mass 1.5 global filtered illumination, of particular notefor this device is the measured air stability. Subsequent work hasfocused primarily on alternative material systems such as PbS and PbSe,and the dip coating of nanocrystal films via ligand exchange, with aneye toward harnessing carrier multiplication for high efficiency solarcells.

SUMMARY OF DISCLOSURE Description

This disclosure concerns a method for the fabrication of a solar cell onan opaque, non-conductive solid substrate, where all of the componentsof the device can be deposited using a spray-based solution process.Critical elements are found in both the method of deposition as well asin the unique architecture of the cell.

DESCRIPTION OF THE DRAWINGS

The following description and drawings set forth certain illustrativeimplementations of the disclosure in detail, which are indicative ofseveral exemplary ways in which the various principles of the disclosuremay be carried out. The illustrated examples, however, are notexhaustive of the many possible embodiments of the disclosure. Otherobjects, advantages and novel features of the disclosure will be setforth in the following detailed description when considered inconjunction with the drawings.

FIG. 1: Schematic of an all-inorganic nanocrystal-based PV device.

FIG. 2: Effect of annealing CdSe nanorod films at various temperatures.No CdCl₂ was used in these samples.

FIG. 3: (a) SEM image of the top surface of a CdTe film prepared usingthe described spray procedure, and (b) profilometry data for the samefilm.

FIG. 4: Optical microscope image of a Au grid patterned on top of asprayed CdTe nanocrystal film by standard lift-off process.

FIG. 5: Schematic of the inverted heterostructure referenced in thisdisclosure.

FIG. 6: Performance data for an inverted heterojunction PV device.

DETAILED DESCRIPTION OF THE INVENTION

This disclosure concerns a method for the fabrication of a solar cell onan opaque, non-conductive solid substrate, where all of the componentsof the device can be deposited using a spray-based solution process.Critical elements are found in both the method of deposition as well asin the unique architecture of the cell.

Common to all of the devices discussed in the background section hereinis a need to fabricate the structures on a transparent conductivesubstrate, such as indium-tin oxide (ITO) coated glass substrate,followed by deposition of a metal top contact through thermal or e-beamevaporation once the active layers are processed. Furthermore, mostdevices in the literature are fabricated through a spin coating processthat deposits material inefficiently and is not amenable to irregular orarbitrary-shaped substrates. Dip coating has also been examined as analternative, and while more efficient in terms of material usage, itrequires large volumes of solution to be prepared as the substrate sizeincreases. In practice, the nanocrystal deposition solution also has alimited shelf-life due to aggregation caused by cross-contamination withthe organic ligands used for the exchange.

Alternatively, we have found spray application of nanomaterials viaairbrush to compare favorably with these techniques, although surfaceroughness is generally increased while the overall uniformity isreduced.

A major advantage, however, is the potential of using non-standardsubstrates which are not compatible with either spin or dip coating. Inaddition, the material deposition efficiency is much higher. For a 500nm film deposited on a 25×25 mm substrate, the spray coating is aboutfour times more efficient than spin coating.

In order to enable this vision—the deposition of a complete solar cellon any solid surface using only sprayed material solutions—severalcritical elements must be addressed in both the processing and design ofthe device. This disclosure outlines methods to address these issues,overcomes the problems associated with the currently used methods as inprior art, and demonstrates a better method and product than previouslyknown.

Example 1

Spray Deposition Processing

Currently, the ligand shell that surrounds the nanocrystals followingsynthesis promotes solubility of the material and facilitates thesolution deposition process, but serves as a barrier to electrontransport. In order to obtain active layers with sufficient carriermobility to promote charge transport in the device, the ligands presenton the as-prepared materials must be removed or reduced in size.

Ligand exchange and thermolysis are two strategies for accomplishingthis task thereby increasing the carrier mobility of the depositedfilms.

Specific to cadmium chalcogenide materials, a 400° C. sinteringtreatment coupled with CdCl₂ exposure has been shown to also increasethe grain size, further improving transport in the film. A pitfall withthis approach is the film damage that can occur due to loss of theorganic volume from the deposited material, primarily through crackingand pinhole formation. FIG. 2 illustrates this, and shows SEM images ofCdSe nanorod films heated to various temperatures. At 500° C., theindividual nanostructures are no longer visible, and instead a porousfilm is formed. The pinholes and cracks present can lead to shorts whena metal contact is placed on top of the film, particularly if thematerial layer is kept thin.

To avoid this problem, multiple layers can be deposited to fill in thegaps and correct or “fill-in” these film defects as they form. Ligandexchanged films prepared through dip coating rely on this process fortheir continuity, which has also been adapted to spin coating. Recently,an iterative approach to this process was reported where a thin layer ofCdTe nanocrystals are deposited through spin coating, followedimmediately by a 10 sec sintering treatment. The process is thenrepeated for the next layer, which fills in any defects in the film andslightly increases the overall thickness.

In practice, while this process produces high quality films, it istedious and requires 10 cycles to produce an ˜500 nm thick film.

Here, we have incorporated this strategy into a new spray coatingprocess, and found it to also produce high quality material filmssuitable for devices while depositing thicker layers during each cycle.Importantly, substrates can also be coated in parallel with thisprocess, unlike spin coating which is inherently serial. Thisdramatically reduces the time necessary for the preparation of multiplesamples.

This new spray procedure is conducted as follows.

A 40 mg/ml concentrated stock solution of CdTe or CdSe nanocrystals,previously subjected to overnight ligand exchange with pyridine, isdiluted in CHCl₃ to a concentration of 1 mg/mL.

This solution is then applied to a vertically mounted 80° C. substratevia airbrush with a steady pressure of 20 psi N₂ as the carrier gas.

When ˜¼ of the solution has been applied, the sample is removed, driedfor 2 min at 150° C., then dipped in a saturated solution of CdCl₂ inMeOH held at 60° C.

Following this dip, the substrate is immediately washed two times withfresh iPrOH, blown dry with N₂, and placed on a 400° C. hot plate in airfor 1 min.

The sample is then remounted and the next ¼ of solution appliedfollowing the same protocol. After a total of four cycles, the film is˜500 nm in thickness.

The final thickness can be controlled either by reducing the number ofsteps, or by adjusting the concentration of the stock solution.

A second material can easily be introduced into the structure to form aheterostructure at any time by simply substituting a different spraysolution prepared at a similar concentration.

The device measured in FIG. 5 (vide infra) was prepared in this fashion.

It is worth noting that functional devices can be produced withprocessing temperatures lower than 400° C. using this procedure. Theperformance of these samples can decrease as the temperature is reduced,however. Several strategies for lowering the temperature of thisprocessing step while maintaining performance include small moleculeligand exchange and the inclusion of inorganic nanowires.

FIG. 3a shows an SEM image of the top surface of the CdTe following thisprocedure. In addition to the increase in grain size to approximately200 nm, large surface roughness features can be seen. These areillustrated more profoundly in the profilometry data in FIG. 3b . Whiletraditionally roughness is thought to be detrimental to deviceperformance, in the case of these sintered all-inorganic devices weunexpectedly find the impact to be limited.

Table 1 shows data comparing single layer CdTe Schottky-barrier solarcells (glass/ITO/CdTe/Ca/Al) fabricated from both spin and spraycoating.

TABLE 1 Summary of Schottky PV device data comparing spin and spraycoating Average RMS thickness roughness J_(sc) V_(oc) FF Eff. Device(nm) (nm) (mA/cm²) (V) (%) (%) CdTe Schottky (spin) 435 3.9 13.2 ± 0.20.53 ± 0.02 43.1 ± 4.2 3.0 ± 0.3 CdTe Schottky (spray) 510 202 13.3 ±3.0 0.39 ± 0.06 45.7 ± 3.4 2.3 ± 0.3

Despite the substantial increase in surface roughness, overall theperformance of the sprayed device is very similar to the traditionalspin coated sample.

The processing advantages noted above are such that these minordifferences could be tolerable for many applications, and the increasedroughness could possibly benefit some structures through increased lighttrapping.

Furthermore, it is worth noting that the films resulting from this sprayprocess are able to tolerate additional processing steps without damage.

Sprayed CdTe and CdSe films were robust enough to withstand a standardphotolithography lift-off process consisting of Microposit® S1800 seriesphoto resist, LOR™ lift-off resists, MF-319 developer, Nano™ remover PG,acetone and iso-propanol.

The films displayed no observable film-shrinkage, peeling, or warpingduring this process. An optical microscope image of Au patterned onsprayed CdTe is shown in FIG. 4.

Example 2

Architecture

While the spray procedure outlined in the previous section is applicableto a variety of materials and surfaces beyond those described in theExample, further changes to the device architecture are required toenable solar cell fabrication on non-conductive substrates.

A schematic of the proposed inverted heterostructure of such a device isshown in FIG. 5. Note that unlike FIG. 1, this device is not fabricatedon a conductive substrate but rather on any solid, non-porous surfacethat can support the layers.

As this device is constructed “upside-down” it necessitates thedeposition of the metal contact first as the initial layer. Activemetals with low work functions such as Al or Ca are often used inSchottky-barrier solar cells based on nanocrystals to enhanceperformance. Due to their reactivity, however, solution deposition ofthese metals in nanocrystalline form is not an option under ambientconditions.

High work function, noble metals such as Au, Pt, and Ag are far morestable and resistant to oxidation, and solution syntheses for theirnanoclusters are well known. Conductive Ag paint is also commerciallyavailable. The deposition of Au nanoclusters can also be accomplishedvia airbrush, and thermal treatment above 200° C. results in theformation of a metallic Au film on the non-conductive substrate ofinterest. It is also possible to consider use of highly conductivepolymers or transparent conducting oxides in place of the metal; if thesubstrate is also transparent this would produce a “semi-transparentsolar cell”.

Using a higher work function conductive contact then necessitates areversal in the position of the nanocrystal active layers of the devicein order to improve the energy level alignments at the materialinterfaces. Note that the FIG. 5 structure places the CdTe layerdirectly in contact with the metal, unlike in FIG. 1 where the CdSeoccupies this position.

This reversal is critical for fabricating a functioning solar cellentirely through a spray coating process, as it enables the use of lessreactive noble metals as the back contact.

FIG. 6 shows the current density-voltage characteristics (J-V) under airmass 1.5 global conditions for devices employing this invertedheterostructure. Of particular importance is the excellent open-circuitvoltage (V_(oc)) exhibited. Layer thicknesses (in particular the CdSe)have not been optimized in this device and are contributing to the lowoverall efficiency. While one literature report claims to have attemptedthis inverted structure with no success, the authors stated that apossible reason for their failure was shorting through the CdSe layer.Our spray process as described in Section 1 above is critical toavoiding this problem.

Furthermore, it is important to note that while the active layers of thedevice measured in FIG. 6 were processed using spray coating, ITO-coatedglass was used for the substrate and the Au contacts were e-beamevaporated. The device in FIG. 5 requires a low sheet resistance,transparent top contact that can also be spray deposited as the finalstep. Materials such as carbon nanotubes, metal nanowires, or combustionsynthesized ITO are excellent choices in addition to conventionalconducting polymers (e.g. PEDOT:PSS).

The work disclosed here consists of a process and an architecture thatis designed for the spray based deposition of an entirely inorganicdevice on potentially any non-porous substrate of interest. Currentdevice architectures or active layers that rely on low work functionmetal contacts or air sensitive materials cannot be used in this type ofdesign.

In addition, the spray process is much faster and enables multiplesubstrates with a large area to be processed in parallel, dramaticallyreducing fabrication time.

One advantage over previous work is the iterative nature of the sprayprocess, which limits the effect of film defects created duringsintering. The morphology of the film can also be controlled withgreater precision.

This results in pinhole free films suitable for devices despite the highsurface roughness.

Alternatives for solution processed active layers exist (e.g. organicmaterials, other nanocrystal systems) but do not possess the airstability found in the system described above. The iterative sprayapproach is also important for the formation of defect free layers thatsupport charge transport. Finally, the architecture employed is criticalin that it allows the material system to function when all componentsincluding the top and bottom contact are applied using a spraydeposition process.

The above examples are merely illustrative of several possibleembodiments of various aspects of the present disclosure, whereinequivalent alterations and/or modifications will occur to others skilledin the art upon reading and understanding this specification and theannexed drawings. In addition, although a particular feature of thedisclosure may have been illustrated and/or described with respect toonly one of several implementations, such feature may be combined withone or more other features of the other implementations as may bedesired and advantageous for any given or particular application. Also,to the extent that the terms “including”, “includes”, “having”, “has”,“with”, or variants thereof are used in the detailed description and/orin the claims, such terms are intended to be inclusive in a mannersimilar to the term “comprising”.

What we claim is:
 1. A method of spray deposition for inorganicnanocrystal solar cells consisting of: subjecting a first solution ofCdTe or CdSe nanocrystals to ligand exchange with a small coordinatingmolecule; diluting the first solution in solvent to form a secondsolution; wherein the first solution of CdTe or CdSe nanocrystals isdiluted and forms the second solution which has a concentration lowerthan the first and is 1 mg/ml; applying the second solution to asubstrate; wherein the second solution is applied to the substrate thatis vertically mounted and at a temperature of 80° C. and applied viaspraying with a carrier gas pressure of 20 psi; drying the substrate;dipping the substrate in a solution in MeOH of a compound that promotessintering; wherein the compound that promotes sintering istrimethylsilyl chloride; washing the substrate with iPrOH; drying thesubstrate with N₂; and heating the substrate and forming a film on thesubstrate; repeating the steps of applying again the second solution tothe substrate; drying the substrate; dipping the substrate in thesolution in MeOH of the compound that promotes sintering; wherein thecompound that promotes sintering is trimethylsilyl chloride; washing thesubstrate with iPrOH; drying the substrate with N₂; and heating thesubstrate.
 2. The method of spray deposition for inorganic nanocrystalsolar cells of claim 1 wherein the spraying comprises airbrush andwherein the carrier gas is N₂.
 3. The method of spray deposition forinorganic nanocrystal solar cells of claim 1 wherein the first solutionof CdTe or CdSe nanocrystals is a 40 mg/ml solution and furtherincluding the step of utilizing an air brush for the step of applyingthe second solution to the substrate.
 4. The method of spray depositionfor inorganic nanocrystal solar cells of claim 1 wherein the substrateis dried, following the step of applying the second solution to thesubstrate, for about 2 minutes at about 150° C. and wherein the solutionof the compound that promotes sintering in MeOH is at about 60° C. 5.The method of spray deposition for inorganic nanocrystal solar cells ofclaim 4 wherein the step of heating is for about 1 minute.
 6. The methodof spray deposition for inorganic nanocrystal solar cells of claim 5wherein after about two or more cycles of applying the second solutionto a substrate, drying the substrate, dipping the substrate, washing thesubstrate, drying the substrate and heating the substrate, the film isabout 500 nm in thickness.
 7. The method of spray deposition forinorganic nanocrystal solar cells of claim 1 wherein the solutioncomprises CdSe nanocrystals diluted in CHCl₃ to a concentration of 1mg/mL.